Russian-backed separatists opened fire across the line of control with Ukrainian forces this morning, hitting a nursery school and injuring three people.
In response to the incident, Mr Johnson said: “We fear very much that that is a thing we will see more of over the next few days."
US President Joe Biden reiterated the claims, adding: "There is every indication Russia is prepared to go into Ukraine."
Mr Biden went on to allege that the invasion threat is "very high", but said he had no plans to call Vladimir Putin.
Boris Johnson during his visit to Royal Air Force Station Waddington today
Carl Recine
The attack on the city of Stanytsia Luhanska blew a hole through the wall of Kindergarten Number 21.
Classes had already started when the building in Depovska Street was hit by a shell.
Horrific footage shows debris strewn over a play area.
But Mr Johnson insisted the West must stand firm, despite the threat of conflict.
He added: “What we’ve got to do, and I’ve said this before, is we’ve got to end the dependence of the West, and Europeans in particular, on Russian hydrocarbons.
“We can’t be blackmailed this way by Vladimir Putin – we’ve got to end that.
“I just want to say one thing finally about what’s happening in Ukraine – there is still time for the Putin regime to step back.
“There is still time to avoid a catastrophe, a catastrophe for Russia, a catastrophe for Ukraine and for the world.
“If Russia were so mad as to invade, I don’t think people should imagine that this would be a brief business.
“This would be a bloody and protracted conflict in which, I’m afraid, there will be many casualties and including many Russian casualties. I just hope that people in Russia can see that.”
Foreign Secretary Liz Truss, on a visit to Kyiv today, reiterated the Prime Minister's claims.
She said: "Reports of alleged abnormal military activity by Ukraine in Donbas are a blatant attempt by the Russian government to fabricate pretexts for invasion.
Foreign Secretary Liz Truss
Aaron Chown
"This is straight out of the Kremlin playbook.
"The UK will continue to call out Russia’s disinformation campaign."
NATO allies have accused Moscow of misleading the world with “disinformation” by saying it was returning troops to bases, charging that Russia has instead added as many as 7,000 more soldiers near the Ukrainian border.
It comes after the Kremlin expelled US deputy ambassador Bartle Gorman this afternoon, widening the rift between East and West yet further.
Speaking in Kyiv this afternoon, Ms Truss said Russia would secure its status as an "international pariah" if war breaks out.
The Foreign Secretary praised the “fortitude” of the Ukrainian people during her visit.
Speaking at a press conference, she said: “I’m here today to show the United Kingdom’s steadfast support for Ukraine.
“I’m very impressed that, despite the Russian aggression towards Ukraine, that life here in Kyiv goes on and I am impressed by the level of stoicism, both from the Ukrainian people and the Ukrainian government.
“We admire your fortitude in the face of Russian aggression and we stand with you in defence of your right to determine your own future.”
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